Environmentally-driven, regulatory pressure concerning motor gasoline (“mogas”) sulfur levels have resulted in the widespread production of less than 50 wppm sulfur mogas in 2004, and levels below 10 wppm are being considered for later years. In general, this will require deep desulfurization of refinery naphtha streams. The largest target of naphtha streams for such processes are those resulting from cracking operations, particularly those from a fluidized catalytic cracking unit which comprise a large volume of the available refinery blending stock as well as generally higher sulfur content than the “non-cracked” refinery naphtha streams. Naphthas from a fluidized catalytic cracking unit (“cat naphthas”) typically contain substantial amounts of both sulfur and olefins. Deep desulfurization of cat naphtha requires improved technology to reduce sulfur levels without the severe loss of octane that accompanies the undesirable hydrogenation of olefins.
Hydrodesulfurization is one of the fundamental hydrotreating processes of refining and petrochemical industries. The removal of feed organically-bound sulfur by conversion to hydrogen sulfide is typically achieved by reaction with hydrogen over non-noble metal sulfided supported and unsupported catalysts, especially those containing Co/Mo or Ni/Mo. This is usually achieved at fairly severe temperatures and pressures in order to meet product quality specifications, or to supply a desulfurized stream to a subsequent sulfur sensitive process.
Olefinic naphthas, such as cracked naphthas and coker naphthas, typically contain more than about 20 wt. % olefins. Conventional fresh hydrodesulfurization catalysts have both hydrogenation and desulfurization activity. Hydrodesulfurization of cracked naphthas using conventional naphtha desulfurization catalysts under conventional startup procedures and under conventional conditions required for sulfur removal, typically leads to an undesirable loss of olefins through hydrogenation. Since olefins are high octane components, it is desirable to retain the olefins rather than to hydrogenate them to saturated compounds that are typically lower in octane. This results in a lower grade fuel product that needs additional refining, such as isomerization, blending, etc., to produce higher octane fuels. Such additional refining, or course, adds significantly to production costs.
Selective hydrodesulfurization to remove organically-bound sulfur, while minimizing hydrogenation of olefins and octane reduction by various techniques, such as selective catalysts and/or process conditions, has been described in the art. For example, a process referred to as SCANfining has been developed by Exxon Mobil Corporation in which olefinic naphthas are selectively desulfurized with little loss in octane. U.S. Pat. Nos. 5,985,136; 6,013,598; and 6,126,814; all of which are incorporated herein by reference, disclose various aspects of SCANfining. Although selective hydrodesulfurization processes have been developed to avoid significant olefin saturation and loss of octane, such processes have a tendency to liberate H2S that reacts with retained olefins to form mercaptan sulfur compounds by reversion.
As these refinery hydrodesulfurization catalytic processes are operated at greater severities to meet the lower sulfur specifications on products, the H2S content in the process streams increases, resulting in higher saturation of olefins and reversion to mercaptan sulfur compounds in the products. Therefore, the industry has sought for methods to increase the desulfurization efficiency of a process while reducing or eliminating the amount of reversion of mercaptan sulfur compounds in the final product.
Many refiners are considering combinations of available sulfur removal technologies in order to optimize economic objectives. As refiners have sought to minimize capital investment to meet low sulfur mogas objectives, technology providers have devised various strategies that include distillation of the cracked naphtha into various fractions that are best suited to individual sulfur removal technologies. While economics of such strategies may appear favorable compared to a single processing technology, the complexity of overall refinery operations is increased and successful mogas production is dependent upon numerous critical sulfur removal operations. Economically competitive sulfur removal strategies that minimize olefin saturation and minimize the production of mercaptan sulfur compounds (also referred to as “mercaptans”) in the products, as well as decrease the required capital investment and operational complexity will be favored by refiners.
Consequently, there is a need in the art for technology that will reduce the cost and complexity of hydrotreating olefinic naphthas to low levels of sulfur content while either reducing the amount of mercaptans formed or by providing an economical process to destroy the mercaptans that are formed as a resultant of the hydrotreating process. There is a need in the industry for a process to reduce these product mercaptan levels while meeting higher sulfur reduction specifications, minimizing the saturation of olefins, and reducing the loss of octane in the final product.